Variable axis locking mechanism for use in orthopedic implants

ABSTRACT

A locking mechanism assembly to allow locking of a fastener in an orthopedic implant having its axis at a variable angle relative to the axis of threaded through opening in the implant. The assembly includes a locking insert, having threads which mates with the threads of the through opening and which includes a smooth opening that forms a torque driving recess and an annular flange. A fixator has a head portion with external threads of a harder material than that which forms the through opening so that the threads of the fixator will deform the insert to lock the fixator in a desired position relative to the plate.

This application is a continuation application of U.S. application Ser. No. 12/804,062, filed on Jul. 13, 2010, which claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/270,980, filed on Jul. 15, 2009, herein fully incorporated by reference.

FIELD OF THE INVENTION

This present invention relates to a mechanism for allowing a screw or peg to be used in an orthopedic implant with the long axis of the screw at a variable angle to the orthopedic implant and subsequently to be locked into a desired orientation. The mechanism comprises an assembly, which is used in a threaded hole in the plate that would otherwise provide for a locking relationship with a locking screw at a fixed angle. Thus the assembly provides the option of either fixed angle locked fixation or locked variable angle fixation using the same plate hole.

BACKGROUND OF THE INVENTION

The present invention provides an advance in the design of an assembly which allows a fixator or fastener, including for example, a screw, or peg, to be inserted through a threaded hole in a stabilizer or implant, such as a plate, anchor or cage, at a variable angle in order to best capture a bone or bone segment with the fixator. The angle of the fixator can subsequently be locked to fix the bone or bone segment relative to the plate, or to fix the plate relative to the bone or bone segment. The invention allows for at least about 10°, and more preferably 15°, of angulation relative to a longitudinal axis of the opening that the fixator is inserted through. The assembly provides the advantage of providing the option to the surgeon of using a locking screw having a threaded head which mates with the threads in the hole to fix the screw at a pre-determined angle, or to use the assembly which comprises a threaded bushing in the hole with a threaded screw at a different desired angle relative to the hole.

There are numerous implant applications that can benefit from a variable angle locking mechanism. Specific examples include use in the small bones, i.e. those bones distal to the elbow and knee and the clavicles, although it is understood, that the mechanism can also be of use in other areas of the body, including the long bones, the pelvis and the spine.

SUMMARY OF THE INVENTION

The assembly of the present invention includes a locking insert that is threaded into, and thus mechanically seated in, the internally threaded opening in an implant. The locking insert has an annular flange or shoulder that surrounds the opening in the implant on the superior surface (relative to the associated bone or bone fragment), and also has a central through opening that is preferably smooth and hexagonal in cross section to allow the insert to be screwed into the plate opening. The locking insert is made from a biocompatible material that can be deformed by the threads of the fixator head, (i.e. a screw or peg,) so that the fixator is inserted through the locking insert and into the adjacent bone to the point where the proximal head threads interact with the internal opening of the locking insert to cause the material to flow and accept the head of the fixator at any angle (up to ˜15°) while maintaining a rigid construct. Both the minor diameter and the major diameter of the screw head include a taper to improve the locking within the locking insert. Preferably, the major diameter of the threads taper out at a larger angle than the minor diameter, i.e. by a difference of from about 2° to about 20°, preferably from about 5° to about 15°, and preferably about 8° to about 12° degrees of difference, so that the threads widen at a greater rate than the head does to improve the feeling when the fixator is locked into the insert.

FIG. 1 is an exploded view of a variable locking assembly in accordance with the present invention with the plate illustrated in section;

FIG. 2 is a view from the top and the side of the variable locking assembly of the present invention;

FIG. 3 is a side view of the screw head of FIG. 1;

FIG. 4 is a top side view of variable locking insert of FIG. 1;

FIG. 5 is a cross section of the variable locking assembly insert of FIG. 2 taken along line 5-5; and

FIG. 6 is a cross section of the plate shown in FIG. 1 with a threaded locking screw.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of a variable axis locking mechanism assembly in accordance with the present invention which are further illustrated in FIGS. 2 through 6. FIG. 5 shows a cross section of the assembly of FIG. 1. The assembly 10 includes an implant member which is shown as a plate 12, a locking insert 16 and a variable locking fixator, which is shown as a screw 20. For the purpose of this description, the plate is shown as part of an annular portion of a plate including a through hole 22 with internal threads 24, and optionally grooves 26 that act as seats for a drill guide having mating cylindrical projections. The plate could have any appropriate shape according to the application with which it is used, such as distal radius plates, calcaneal plates, long bone plates, plates for the clavicles, spinal plates, plates for use in the hand and foot, or any other surgical implant.

The plate generally has a top side, or superior side, i.e. the side which faces away from the bone in use, with a generally constant through thickness to a bottom side, or side that faces toward the bone in use. The plate can be planar, or have another topography, according to the application, although the through hole portion 22 must have a topography that allows the through hole of being capable of receiving the locking insert 16 and the threads 24 of the hole are capable of mating with the external threads 32 of the body 34 of the locking insert or the head of a fastener designed for fixed angle locking. As illustrated in FIG. 4, the locking insert includes an annular flange member 36, which generally forms a shoulder area that seats against the top portion of the plate that surrounds the through hole 22. The locking insert further includes an internal opening 38 that is advantageously smooth and which has a cross-sectional configuration that allows the locking insert to be screwed into the plate hole. Preferably, this is a hexagonal shape having internal edges where the flats join that are smoothed out to better accommodate the screw head. The locking insert is made from a biocompatible material that is softer than the screw so that when the screw head is screwed into the locking insert, the external threads will cause the surface of the internal opening 38 to flow or deform to form threads in the locking insert, causing the screw to lock into position in the locking insert relative to the plate.

FIGS. 1, 2 and 3 show a variable axis locking screw 20 which could also be a variable axis locking peg. The screw includes a portion having a thread 40 for attachment within a bone or bone segment. The screw can include an insertion tip 42 that has a point, or as shown, a blunt tip with optional cutting flutes 44. The screw has a head portion 46 that is joined to the distal threaded portion by an area 48 having threads of a smaller major diameter and also including an area that is free from threads or is cylindrical. The head of the screw includes external threads 50 where the minor diameter and major diameter both taper, but preferably, the major diameter tapers at a smaller angle than the minor diameter so that the threads become thicker as they progress toward the top end of the screw. The minor diameter tapers at an angle of from about 20° to about 60°, and preferably from about 30° to about 50° degrees and most preferably about 35° to about 45° while the major diameter tapers at an angle of from about 30° to about 70°, and preferably from about 40° to about 60° degrees and most preferably about 45° to about 55° with an advantageous differential being about 8° to about 12°. The head portion 46 further includes a torque driving recess 52, with an optional bore 80 which retains the screw 54 on the post of a screwdriver.

FIG. 6 illustrates the plate of the present invention accepting a locking screw which has threads on the exterior surface of the head that mate with the internal screws of the through hole in the plate.

While in accordance with the patent statutes the best mode and preferred embodiment have been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims. 

What is claimed is:
 1. A variable angle locking mechanism assembly for use in an orthopedic implant including an longitudinal axis and a through opening defining an internal wall with threads and an external surface, comprising: a locking insert having external threads that mate with the threads of the through opening of the implant, and a fixator having a portion for attachment in bone, and a head portion having external threads, the internal wall of the locking insert being of a material that will deform relative to the material of the external threads of the head portion of the fixator so that the screw head will seat in the locking insert in a locked relation to the implant.
 2. The variable angle locking mechanism assembly as set forth in claim 1, wherein the implant is a plate and the opening in the plate can optionally accommodate a fixed angle locking screw or locking insert.
 3. The variable angle locking mechanism assembly as set forth in claim 1, wherein the insert further includes an annular flange about the through opening which forms a shoulder that mates against an external surface of the implant.
 4. The variable angle locking mechanism as set forth in claim 1, wherein the locking insert is made of peek and the fixator is made of a metal.
 5. The variable angle locking mechanism assembly as set forth in claim 4, wherein the through opening in the locking insert forms a torque driving recess.
 6. The variable angle locking mechanism assembly as set forth in claim 5, wherein the through opening has a cross-section that includes flat surfaces.
 7. The variable angle locking mechanism assembly as set forth in claim 6, wherein the through opening has a cross-section that forms a polygon having from 4 to 10 sides.
 8. The variable angle locking mechanism assembly as set forth in claim 7, wherein the polygon has from 5 to 8 sides.
 9. The variable angle locking mechanism assembly as set forth in claim 8, wherein the polygon is a hexagon.
 10. The variable angle locking mechanism assembly as set forth in claim 1, wherein the threaded portion of the head has a minor diameter which has a taper and a major diameter which has a taper.
 11. The variable angle locking mechanism assembly as set forth in claim 10, wherein the minor diameter which has a taper which is larger than the taper of the major diameter.
 12. The variable angle locking mechanism assembly as set forth in claim 11, wherein the difference in the taper of the major diameter to the minor diameter is from about 5° to about 15°. 